1. Field of the Invention
This invention relates generally to a tracking servo system of a magnetic disc apparatus and, more particularly, is directed to a tracking servo system for a disc memory of a magnetic disc apparatus in which servo information is obtained from a magnetic track formed on a magnetic disc and this servo information is used to accuractely position the magnetic transducer head of the apparatus.
2. Description of the Prior Art
Generally, in a tracking servo system for a magnetic disc of a magnetic disc apparatus, a control operation for positioning a transducer head is carried out mainly for the purpose of the following two points. That is, one of them is to carry out a so-called seek operation in which the transducer head is moved from one track to another track as soon as possible, while the second one is to enable the transducer head to follow the track as accurately as possible after the transducer head has moved regardless of an external disturbance, in other words, to carry out the tracking operation with high accuracy.
To carry out these control operations, it is necessary to provide detecting means for detecting servo information concerning the position and moving speed of the transducer head, etc. On the basis of the differences of the detecting means themselves, there are known the following three kinds of servo systems in the prior art.
The first servo system is an optical scale servo system which comprises a first optical scale fixed to a chassis and having slits corresponding to a plurality of tracks, a second optical scale having slits corresponding to a plurality of tracks and movable on the first optical scale in association with the transducer head and optical coupling means formed of a light emission portion and a light receiving portion located above and below the first and second optical scales, and in which when the second optical scale enters the optical path of the optical coupling means, the light amount of the light receiving portion is changed in response to the displacement amount of the second optical scale and the displacement amount is converted to and delivered as an electrical signal. The second servo system is such a servo surface servo system for a magnetic disc apparatus having a plurality of discs supported by a single shaft, in which the whole of a particular disc of the plurality of discs is employed for servo use. Then, the third servo system is an embedded type servo system in which a servo information is provided between sectors of each track recorded on the disc.
However, the respective prior art systems as mentioned above have the following defects. Specifically, in the case of the optical scale servo system, since the position of the head to be controlled and the position of the optical scale used as the detecting means are distant from each other, the optical scale servo system is weak or easily affected by the thermal expansion. The reason is that since the temperature distribution within the magnetic disc apparatus is not uniform, the temperature is different in the optical scale and the disc, and further, even when they are equal in temperature, the thermal expansion coefficient is different in the optical scale and the magnetic disc because they are made of different materials. In general, it is quite difficult to correct such difference.
The servo surface servo system requires a special disc surface for the servo operation. Particularly in the case of the magnetic disc apparatus which can not afford so many dics, the servo disc reduces the number of available data discs, thus reducing the storage capacity for data. Further, in the case of the embedded servo system, the servo information is obtained once per revolution of the disc so that troubles will occur from a head positioning precision standpoint.